The need to remain competitive in cost and performance in the production of semiconductor devices has caused a continuous increase in device density of integrated circuits. To accomplish higher integration and miniaturization in a semiconductor integrated circuit, miniaturization of a circuit pattern formed on a semiconductor wafer must also be accomplished.
Design rules define the space tolerance between devices or interconnect lines so as to ensure that the devices or lines do not interact with one another in any unwanted manner. One important layout design rule that tends to determine the overall size and density of the semiconductor device is a critical dimension (CD). A critical dimension of a circuit is defined as the smallest width of a feature, e.g., a line or the smallest space between two lines. Another critical design rule is minimum pitch, which is defined as the minimum width of a given feature plus the distance to the adjacent feature edge.
The continual reduction in CDs places increasing demands on the techniques used to form the features. For example, photolithography is commonly used to pattern these features. Typically, photolithography involves passing radiation (light) through a reticle and focusing the radiation onto a radiation sensitive photoresist material. By directing radiation through the reticle, the pattern in the reticle may be focused on the photoresist. The radiation causes a chemical change in the illuminated parts of the photoresist, which allows those illuminated parts to be selectively retained or removed, depending upon whether positive or negative photoresist is used, relative to parts which were non-illuminated. Thus, the exposed (illuminated) and unexposed (non-illuminated) parts form a pattern in the photoresist.
Because photolithography is typically accomplished by projecting radiation onto a surface, the ultimate resolution of a particular lithography technique depends upon factors such as the optical proximity effects and the wavelength of the radiation used. Optical proximity effects are known to result from optical diffraction in the projection system. The diffraction causes adjacent features to interact with one another in such a way as to produce pattern-dependent variations. Accordingly, the closer together features are, the more proximity effect is seen. Thus, the ability to locate line patterns close together encroaches on optical parameter limitations.
One proposed solution for achieving critical dimensions and pitches beyond those currently achievable by photolithography techniques alone is utilizing directed self-assembly (DSA) lithography, which exploits the propensity of certain block copolymers to self-assemble into ordered morphologies, having spherical, cylindrical, lamellar, or bicontinuous gyroid microdomains. However, one challenge for incorporating DSA lithography into mass production is increased processing time caused by long annealing times required for a block copolymer to self-assemble into suitable CD targetry and uniformity.
Accordingly, new and improved methods of patterning semiconductor devices using DSA lithography are needed.